Modular steam generator



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United States Patent [72] Inventor Baldassare Russo, Jr.

King County, Washington (16608 Marine View Drive SW, Seattle, WA 98166)[21 Appl. No. 753,366 [22} Filed Aug. 19, 1968 [45] Patented Dec. 1,1970 [54] MODULAR STEAM GENERATOR 17 Claims, 6 Drawing Figs.

[52] 0.8. CI 122/336, 122/481 [51] lnt.Cl F22b3l/00 [50] Field ofSearch122/235, 336, 478, 481

[56] References Cited UNITED STATES PATENTS 2,988,063 6/ 1961 Vorkauf122/336 3,003,482 10/1961 Hamilton et al. 122/478 3,012,548 12/1961Guszmann 122/235 3,020,894 2/1962 Rowand 122/481 PrimaryExaminer-Kenneth W. Sprague Attorney-Christensen, Sanborn & MatthewsABSTRACT: A steam generator arrangement is provided whereby variouscomponents may be independently designed and positioned to reduce oreliminate intercomponent dependency disadvantageously affectingefficiency and economy, and enabling component standardization andprefabrication with minimized field assembly. The key structuralfeatures pertain to elimination of direct connection of boiler tubes todrums, with the boiler section comprising modular boiler units includingheaders in the flow path between the steam drum and the boiler tubes.The lower mud drum is completely eliminated and the steam drum ispositioned exteriorly of the combustion gas path.

Patented Dec. 1, 1970 Sheet 1 of 3 INVENT OR BALDASSARE RUSSO, JR.

EPRS SQ. FT.

Patented Dec. 1, 1970 3,543,733

Sheet 2 FIG. 4.

1 2 34567a91011121314151617181920 UNIT SERIES NUMBER r-4 INVENTOR J3BALDASSARE RUSSQJR 38 Patented Dec. 1, 1970 3,543,733

Sheet of s USER'ES F|G.5. A B

FURNACE DEPTH, FEET FURNACE WIDTH STEAM ENERATOR CONVEC'HON ASS WIDTH UWlDTH 2 LLNJTWJDIHJEEI FURI lrACE W H FURNACE DEPTH 28L 16 20 1Q 3 FIG.6.

u 1 4 516 1 20 28R 22 I if J F 14 INVENTOR 36R BALDASSARE RUSSQJR.

d L' x I W 1 MODULAR STEAM GENERATOR The present invention relates tosteam generators, and more particularly to an arrangement wherebycertain steam generator components may be Separately and individuallydesigned and constructed to achieve optimum standards. Morespecifically, the basic concepts of the invention seek to obviatecertain intercomponent dependencies and the structural and operationallimitations associated therewith.

Individual steam generator components are to a significant degreelimited with regard to size, arrangement and opera tionalcharacteristics because ofdependcncy upon other components. The depth orwidth of a steam generator furnace may be as significantly limited anddetermined by factors such as the length of the steam drum as byperformance factors such as fuel characteristics or output requirements.Such intercomponent dependency and limitations, moreover, may extend tobetween other steam generator components in addition to the furnace andthe steam drum. The boiler section and the superheater section, as wellas other steam generator components including other heat recoveryelements, may likewise be subject to limitations because ofintercomponent dependency. The effects and disadvantages of suchinterdependency may impose design and operational limitations whichrequire that individual components be constructed and formulated tooperate at less than optimum efficiency and economy.

Accordingly, it would be of significant advantage if certain undesirableeffects and limitations of intcrcomponent depen' dency in steamgenerators could be eliminated or significantly reduced to enable moreindividual and independent design and construction of steam generatorcomponents; i.e., furnace, boiler section, stear'n drum, superheater,etc., with each component operative at optimum component efficiency andconstructed for optimum economy.

Accordingly, it is an object of the present invention to provide animproved steam generator arrangement reducing or eliminating certaindisadvantageous intcrcomponent dependencies and limitations.

Elimination of certain aspects of component interdependency can alsogive rise to additional heretofore unattainable benefits relating tostandardization and prefabrication of steam generator components. Inaddition to elimination ofthe aforementioned disadvantageouslimitations, the present invention contemplates enablernent oftechniques of steam generator design, manufacture and assembly wherebysteam generator components may be prefabricated as standardized modularelements, with individual prefabricated modular elements being adaptablefor utilization and field assembly in a variety of specific steamgenerator unit designs. Thus, it would be possible, in accordance withthe present invention, to maintain an inventory of prefabricated modularsteam generator components; e.g., I boiler sections, furnaces,superheater, which could be selectively field assembled in accordancewith required steam generator operating characteristics.

Accordingly, it is a further object of the present invention to providea modular steam generator construction wherein components may bestandardized and prefabricated for field assembly.

One of the most significant structural aspects of presently known steamgenerator configurations affecting intercomponent dependency is theconstruction and arrangement of the boiler section. Traditionally,boiler sections are comprised of an upper steam drum and a lower muddrum with boiler tubes extending directly therebetween. Inasmuch asevery boiler tube is directly connected to both the steam drum and thelower drum, the length of the boiler section is determined by the lengthof the drums or vice'versa. This interdependency affects not only thelength of the drums, boiler section, and superheater arrangement, but itmay further affect other structural and operational relationshipspertaining to other steam generator components. For example, dependingupon the specific furnace arrangement and the direction of burnerorientation and flame travel, a certain minimum furnace depth may bemandatory based upon length of flame travel. In some particular steamgenerator arrangements, furnace width or depth will determine steam drumlength which in turn would determine boiler section length. Thus, aparticular boiler section length would become mandatory regardless ofother aspects of the requirements of the unit thereby introducing apotentially adverse affect upon optimum efficiency and economy.

The most important structural aspects of the present invention pertainto the arrangement and construction of the boiler section of a steamgenerator. In accordance with the inven tion, a boiler section may beconstructed in individual modular units each comprising an upper andlower header with boiler tubes extending thcrcbetwcen. The headers maybe appropriately connected to other steam generator components includingthe steam drum thereby eliminating direct connection between the boilertubes and the steam drum and con sequently eliminating theinterdependency between drum length and boiler section length. Furnacewater wall tubes may similarly be routed into headers forinterconnection with the steam drum thereby further reducing componentinterdependency. 7

By a further aspect of the present invention the lower mud drum iscompletely eliminated and the steam drum is located outside of the gaspath thereby effecting further improvement in unit efficiency andeconomy.

A better understanding of the invention may be had by reference to thefollowing detailed description of a preferred embodiment thereoftakcn inconnection with the accompanying drawings wherein:

FIG. 1 is a view in perspective partially schematic and broken away of asteam generator arrangement embodying the principles of the presentinvention;

HO. 2 is a front elcvational vicw partially broken away and in crosssection showing the modular boiler unit of the invention;

FIG. 3 is a side elcvutional view of the modular boiler unit of FIG. 2:

FIG. 4 is a graph which plots furnace Effective Projected RadiantSurface in square feet against Unit Series Number;

FIG. 5 is a graph which may be utilized in conjunction with the graph ofFIG. 4 to obtain certain steam generator parameters based upon aparticular Unit Series Number; and

FIG. 6 is. a schematic plan view of the steam generator of FIG. 1showing the flow path of the combustion gases.

Referring to the drawings, the steam generator of the present inventionis shown as basically comprising a furnace section 10, a superheatersection 12 and 35 and a boiler section I4. Burners 16 mounted in thefront wall 18 of furnace it) are directed rearwardly as indicated byarrows 20 in FIG. 6 and are equipped to provide fuel combustion for thesteam generator, with gases which are the product of such combustionbeing directed into the furnace. As depicted in FIG. 6, the combustiongases travel in the direction of arrows 20 to the rear of furnace 10 andthen through a furnace exit screen 23 into the convection pass'formed bysuperheater 12 when required and boiler section 14 as indicated byarrows 22. The gases leave the unit at the front of the boiler section14 in the direction of arrow 24. Y

The structural details of furnace 10 may be in accordance with wellknown principles that should be apparent to those skilled in the art.Accordingly, the furnace 10, like other well known structural features,is shown schematically as comprising tubular elements 26 which form afront furnace water wall 18, sidewalls 28L and 28R, and a rear wall 30.The furnace roof 32 also comprises water tubes 26, but as will beexplained hereinafter, some of these tubes may be diverted to pass steamtherethrough thereby to form a radiant superheatcr section (whenrequired).

Fecd water is initially supplied to the unit by any known means (notshown) into the steam drum Sit which may be generally constructedutilizing many appropriate known fcatures and which comprises druminternals 52 including liquidvapor separator means well known to thoseskilled in the art.

. 3 Although the steam drum 50, as to many of its structural details, isconstructed in a known manner, several important aspects thereof, suchas the specific location and size, particularly the length, of the drumare very important aspects of the present invention, as will appear fromthe disclosure which follows.

Water flows between the drum 50 and the boiler section 14. Thestructural configuration and arrangement of the boiler section 14 is thekey element of the present invention and gives rise to many of theattendant advantages. As depicted in FIGS. 1, 2, and 3, the boilersection 14 is comprised of modular boiler units 4'10, there being fivesuch units depicted in the steam generator of FIG. 1. Each modular unit49 takes the form of an open-ended box frame comprised of refractorysidewalls ill and an upper header 42 and a lower header 44 with boilertubes 46 extending in flow relationship therebetween. Each upper header42 is connected to the steam drum 50 by a riser tube 33 and a downcomertube 34. Thus, water from drum 50 may flow into the modular units 40 ofboiler section 14 through downcomers 34-, with a flow of heated waterreturning to drum 50 from boiler section 14 through risers 33. The waterfrom the drum 5t flows into headers 42 and then through boiler tubes aswhere it is heated by the heat exchange which taken takes place withcombustion gases flowing through boiler section l4 across tubes 46. Asseen in FIGS. 1, 2, and 3, the open cross-sectional areas of the framesare progressively smaller from one frame to the next in the directionrelatively away from the furnace. Thus the combined area of the narrow,vertically elongated openings between and coextensive with the tubes andthe sidewalls 41 in each unit,.is progressively smaller from one unit tothe next in this direction, so that the mass flow through the units isincreased as the convection path becomes progressively more remote fromthe furnace. Some of the heated water flows back to drum 50 in form ofwater-steam mixture through risers 33 with the remaining portion flowingdown into lower header 44 and through downcomer tubes 38 into thefurnace water wall lower headers 39 from where it flows upwardly intowater wall tubes comprising the furnace walls 18, 28, and 30 for furtherheating by heat exchange with the products of combustion within thefurnace l0.

Headers 54 receive the upilow of steam-water mixture from the furnacewater wall tubes, and transmit this flow to'steam drum 50 throughconnecting tubes 56. in the drum 5%, the flow is passed through steamwater separators and chevron driers (not shown) in any one of the usualmanner, where steam and water are separated, with the water beingrecirculated through the boiler i4 and furnace lti. While the steamafter passing through the chevron drier is directed through superheaterconnecting tubes 58 into a convection superheater inlet header 59 andinto tube wall 36 of superheater enclosure 35, and tubes 6t) ofsuperheatcr 12. The superheater i2 is conventional in structure andarrangement and includes a steam temperature control device 64 which isdepicted schematically in FIG. 1. Superheater i2 is a convectionsuperheater and comprises superheater elements formed in vertical loopsof tubes till which may. be supported from the top by hanging from theroof tubes 336T of the convection pass or by mounting upon the floortubes of the superheater enclosing wall structure (not shown). Thesuperheater elements will, of course, be suitably spaced depending upongas mass flow therethrough, gas temperature, the type of fuel utilizedand other design considerations. The convection superheater 12 isself-contained in enclosure 35 which is suitably insulated includingroof tubes 361" floor tubes not shown, sidewall tubes 36S, and rear wall36R which form an extension of the furnace rear wall 3t When required aradiant superheater may be provided on any of the furnace walls of thefurnace it in FIG. 1 the radiant superheater is provided by utilizingcertain tubes 26 which comprise a part of the furnace roof 32. Aspreviously stated, the furnace water wall tubes 26 convey a mixture ofwater and steam to drum 5% by way of headers 54%. As depicted in H6. l,some of the water wall tubes 26 which form sidewall ZSR are continuouslyintegral with tubes forming root 32 and are con nected directly into aheader 54, However, others of the tubes 26 in sidewall 28R within thatportion of roof that is radiant superheater are connected into anintermediate header 5'5. and disconnected from tubes 26 which form roof32. This sec tion of roof tubes will be also disconnected from header 5dand joined to headers 57 and 59. When it is desired to arrange a portionof the furnace roof 32 into a radiant superheater section, to adjustfinal steam temperature, it is merely necessary to disconnect a desirednumber of the roof tubes as from between headers 54 and wall ZtiR, andreconnect them between auxiliary headers 57 and 59. and upt'lowing tubes26 of wall 28R with header 55, respectively, and vice verse to reduceradiant superheater section. It will be noted that auxiliary headers 55and 57 and the auxiliary forward end of header 59 are shown in dottedform. This is to depict the fact that these headers are not neededunless a radiant superheater section is required. In such a case, header59 may be extended by adding the auxiliary forward section shown dottedand auxiliary headers 55 and 57. Also, a connection 37 between header 55and drum 5%} should be included to provide for flow into the drum of thesteam-water mixture entering 55 from tubes 26; of sidewall ZSR; When noradiant superhcater is required, all the roof tubes as which wouldotherwise be connected between headers 57 and 5% will be integral withsidewall 28R and terminate into header 54 as previously described.

Steam flow into the radiant supcrheatcr header 57 is provided from drum59 by way of connecting tube oil which is the rerouted branch of thetube system 58 which normally feed steam to the convection superhcater12 when radiant superheatcr is not required. Steam from header 5'7 flowsthrough the radiant superheater tubes 26 in the furnace roof 32 toheader 59 where the steam flows through the convection superheaterenclosure 35 and convection superheater l2 .to the steam generator unitoutlet 62.

As previously stated, the most important structural concept of thepresent invention resides in the arrangement of boiler section 14. inthe typical boiler section construction heretofore known, all boilersection tubes are connected directly to an upper steam drum and to alower mud drum, the steam drum being similar to drum 5%. Because everyboiler section tube, in prior art structures, is connected directlybetween two drums, the length of the drums and the length of the boilersection must necessarily be equivalent. Accordingly, a componentinterdependency would exist physically relating steam drum lengthdirectly to boiler section length regardless of whether optimumoperational efficiency would require differing lengths. Furthermore, dueto the number of individual tubes connected directly to the drum, thewall thickness of the drum must be maintained above a particularminimum. Additionally, both drums including the steam drum would have tobe located within the path ofthc combustion gases which must flow acrossthe boiler tubes to effect the necessary heat transfer. This is anadditional factor preventing decrease of the cost and complexity ofthcsteam drurn construction due to the thermal stresses to which the drumis exposed.

By utilization of the present invention, these and other drawbacks maybe overcome or eliminated. As will bc apparent from the foregoingdescription, the necessity for a lower mud drum is completelyeliminated. Furthermore, the steam drum 50 may be located completelyoutside the gas path and its orientation with relation to the boilersection, or other steam generator components, may be varied; i.e., itmay be situated with its length parallel to the length of the boilersection as shown in the drawings, or it may be oriented in some otherconvenient juxtaposition. By arranging the boiler section in accordancewith the present invention there results a significant decrease in thenumber of tubes which must be directly connected to the drum, therebyfurther enabling reduction in the thickness of material required for thedrum. Also, less connections would produce a less complex structure andreduction in stress-producing aspects of the structure. The

boiler section connections 33 and 34 may be located at virtually anylocation on the drum 50 and greater flexibility in locating theseconnections makes possible an arrangement whereby drum internals may bepositioned on both sides of the drum 50 thereby nearly doubling thesteam-water separation capacity per foot of drum length. It will beapparent that this will significantly reduce drum length for a givenrequired capacity thereby further reducing substantially the cost of theunit. These and many other advantages and improvements made possible byutilization of the present invention will be apparent to those skilledin the art.

In addition to the advantages discussed herein and others too numerousto describe in detail, the present invention makes possible the veryimportant and valuable achievement of a steam generator comprisingstandardized modular components. As shown in FIGS. 1, 2, and 3, theboiler section 14 is comprised of modular boiler units 40 which may bemanufactured by prefabrication in standardized sizes. The units 40 maybe constructed to include outer enclosing walls'41 including refractorymaterial or other appropriate materials which would suitably thermallyinsulate the boiler section convection path. Depending upon theparticular operational requirements of a steam generator unit, anappropriate number of suitably sized boiler units 40 could be selectedfrom among a prefabricated inventory of standard sizes. The units 40could then be assembled together utilizing known assembly techniques toconnect the necessary water tubes by welding of the like, and to jointogether the outer wall portions 41 to form a continuous boiler section14 made up of the appropriate number and size of units 40. Of course, itwould be possible to totally field assemble the entire boiler section byonsite interconnection of boiler units 40.

In a similar fashion, other section of the steam generator such as theconvection superheater l2 integral with superheater enclosure 35, steamdrum 50 and the furnace 10, could be prefabricated and/or fieldassembled in accordance with specific requirements, with the possibilitythat these sections could also be provided in a prefabricated inventoryof standardized dimensions. Of course, in any case, the amount anddegree of standardization, prefabrication and field assembly would varywith specific requirements, but it is important to note that due to theconcepts of the present invention a great flexibility in sizing andarranging individual sections has been introduced with a resultingpotential for a maximized prefabrication and a minimized field assembly.The furnace, the boiler section, the steam drum and the superheater maynow'be constructed and arranged with significantly reducedinterdependency thereby enabling greater flexibility in the relativephysical juxtaposition of the components as well as their independentsizing to meet optimum efficiency requirements, and their standardizedprefabrication as modular components adapted for field assembly.

A specific example of standardization of steam generator components andassembly may be described by reference to FIGS. 4 and 5 which aregraphic plots of steam generator characteristics. The approach tostandardization depicted in FIGS. 4 and 5 proceeds upon the basis of aset series of steam generator units with each individual unit in theseries being identified by a unit numerical designation. In the exampledescribed herein, a steam generator series comprising twentydesignations exemplifies the selection of variously sized modularstandardized steam generator elements. In FIG. 4 there is shown a curveplotting the Effective Projected Radiant Surface, in square feet, of thefurnace, referred to herein as EPRS, against the Unit Series Number. Inthe specific standardization technique herein described, it is assumedthat twenty standardized units are sufficient. However, it should beapparent from the description which follows that this number could bevaried depending upon specific requirements con templated.

EPRS is a function of steam generator output requirements and,therefore, from a determination of the output requirements of aparticular installation there may be derived the particular EPRSrequirement. Knowing the square footage of EPRS required, there may bederived from FIG. 4 a particular Unit Series Number from 1 to 20 whichrepresents the particular dimensional characteristics corresponding tothe EPRS requirement.

FIG. 5 relates the Unit Series Number to unit dimensionalcharacteristics, and, accordingly, knowing the Unit Series Number andprojecting the appropriately designated line in FIG. 5 to the left theremay be ascertained the Average Furnace Height in feet as indicated uponthe diagonal line at the left of FIG. 5. Projecting the Unit SeriesNumber to the right in FIG. 5 there may be ascertained: the FurnaceWidth, by projecting vertically downward to the lower horizontal linedesignated Unit Width the intersection of the horizontal Unit SeriesNumber line and the diagonal line labeled A; the Steam Generator Width,by projecting vertically downward to the lower horizontal linedesignated Unit Width the intersection of the horizontal Unit SeriesNumber line and the diagonal line labeled B; the Convection Pass Width,which is the difference between the Steam Generator Width and theFurnace Width; and the Furnace Depth, by projecting the horizontal UnitSeries Number line to the vertical line at the right of FIG. 5.

The data depicted in FIGS. 4 and 5 may be translated into tabular form,and the following Table I exemplifies a steam generator seriescomprising only the three standardized units corresponding to UnitSeries Numbers 1, 9, and 20.

It should be apparent that a more extensive tabulation could be derivedfrom FIGS. 4 and 5 to include additional Unit Series Number designationswhich could extend to twenty or exceed that amount.

Having made a determination of the convection pass width, modular boilerunits 40 can be selected from among a group which would correspond tothe convection pass width involved. The number and size of such modularunits 40 would be determined by the operating requirements of the unit,such as exit gas temperature, but it should be clear that such acomputation and determination may be readily made once required boilercapacity has been determined. Furthermore, other components, such as thesteam drum and superheater, may be sized and arranged in a flexiblemanner to obtainoptimum operational characteristics and adjustmentswould be possible depending upon the specific operational levelsdesired.

For example, the inclusion of a radiant superheater will depend upon thefinal stearn temperature desired, with increased numbers of tubes 26 inroof 32 being diverted for use as radiant superheater tubes for increasein final steam temperature. Furthermore, the adjustability of radiantsuperheater capacity enabled by the ease with which individual tubes 26in roof 32 may be interconnected between either headers 54, 55 orbetween auxiliary headers 57, 59, enables very exact control of finalsteam temperature without variation in furnace exit temperature and mostimportantly without the necessity for altering heating surface area.

Iclaim:

l. A steam generator comprising a furnace,-fuel burner means forgenerating heat in said steam generator by fuel combustion, liquid-vaporflow means in heat exchange relationship with products of saidcombustion, liquid-vapor drum means, said liquid-vapor flow meansincluding boiler means connected to said liquid-vapor drum means with aflow path therebetween, and header means in said flow path intermediatesaid boiler means and said liquid-vapor drum means, said liquid-vaporflow means including water wall tubes forming a roof for said furnacecomprising rooftube header means interconnecting said roof tubes,connecting tubes enabling liquid-vapor flow from said roof tube headermeans to said drum means, auxiliary header means, and auxiliary steamflow connecting means between said drum means and said auxiliary headermeans whereby selective interconnection of particular ones of said rooftubes from between said roof tube header means to between said auxiliaryheader means enables steam flow therethrough from said drum meansthereby providing radiant superheater means.

2. A steam generator according to claim 1 wherein said boiler meanscomprise a plurality of boiler tubes directly interconnecting a pair ofheaders.

3. A steam generator according to claim i wherein said boiler meanscomprise a plurality of separately connected distinctly structuredmodular units, each of said units compris ing a plurality of boilertubes interconnecting a pair of headers with each of said units havingone of said headers connected in flow relationship with saidliquid-vapor drum means.

4. A steam generator according to claim 1 wherein the drum means arecomprised exclusively ofsaid liquid-vapor type.

5. A steam generator according to claim 1 wherein said drum means arelocated outside the path of the products of said fuel combustion.

6. A steam generator according to claim 3 wherein said modular boilerunits are of varying size.

7. A steam generator according to claim 1 wherein said modular boilerunits are of a standardized size selected from among a specificpredetermined number of such standardized sizes.

8. A steam generator according to claim 3 wherein each of said modularunits comprises an upper header having connecting tubes permittingliquid flow from said drum means and connecting tubes permitting liquidflow to said drum means.

9, A steam generator according to claim 8 wherein said liquid-vapor flowmeans include water wall tubes located within said furnace and whereinsaid modular units comprise a lower header with connecting tubespermitting liquid flow from said lower header to said furnace water walltubes.

it). A steam generator according to claim 3 wherein said modular unitsinclude heat insulative outer walls adaptable to interconnect withsimilar walls upon other of said modular units to form a continuous heatinsulative convection gas path in said steam generator.

11. A steam generator according to claim it wherein the number of saidparticular ones of said roof tubes which are connected between saidauxiliary header means may be selectively varied.

l2. A steam generator according to claim it wherein said furnace is of astandardized size selected from among a specific predetermined number ofstandardized sizes.

13. in a steam generator. a furnace for generating a mass of combustionproducts by burning a fuel therein, conduit means having top, bottom andside walls defining an elongated convection path for the passage of thecombustion products in a direction relatively away from the furnace.drum means spaced apart from the conduit means for holding water, andheat transfer means defining a plurality of spaced. parallel boilertubes which are disposed upright in the convection p and interconnectedwith the drum means so as to heat water therein by circulating itthrough the conduit means in heat transfer relationship with thecombustion products, said boiler tubes being arranged in crosswiseplanes of the conduit means at intervals along the length oftheconvection path, and spaced apart from the sidewalls of the conduitmeans so as to form a series of successively.interconnected boiler unitswhich have narrow, vertically elongated openings in the aforesaid planesthereof that are coextensive with the sidewalls of the respective unitsfor the free passage of combustion products therethrough in a coursegenerally pa: lel to the top and bottom walls of the conduit thecombined area of the openings in each respective unit beingprogressively smaller from one unit to the next in the aforesaiddirection relatively away from the furnace, so that the mass flowthrough the units is increased as the convection path becomesprogressively more remote from the furnace.

M. The steam generator according to claim 13 wherein each unit has aplurality of tubes in the plane thereof. and the tubes of eachrespective unit are interconnected with the drum means by a pair ofheaders connected to the upper and lower ends thereof.

The steam generator according to claim l3 wherein the conduit meansincludes a series of open-ended box frames which are disposed cnd-to-endwith one another and have the boiler tubes arranged crosswisetherewithin, the open crosssectional area of the frames beingprogressively smaller from one frame to the next in the aforesaiddirection relatively away from the furnace.

lb. The steam generator according to claim 15 wherein the top and bottomwalls of the frames form headers for the boiler tubes, and the headersare interconnected with the drum means for the supply and return of thewater to and from the tubes.

E7. The steam generator according to claim i5 wherein the box frameshave a common-width but progressively smaller heights in the aforesaiddirection relatively away from the furnace, and there are means forclosing the peripheral gaps between the ends of the frames at eachjuncture therebetween.

